This invention relates to mixer circuits and, more particularly, to mixer circuits operating under limited power supply voltage conditions.
Recent advances in mobile radiotelephony have placed increasingly severe requirements on noise reduction. For example, the GSM standard, which has reduced the separation between adjacent channels to 10 MHz from 20 MHz, requires that the noise 20 MHz away from the carrier (or local oscillator) frequency be down by about 165 dBc/Hz to avoid injecting noise into the adjacent receive channel. One source of noise in radiotelephony arises from the biasing resistors used in the mixer circuitry. In the well-known Gilbert four-quadrant multiplier, for example, disclosed in Gray and Meyer""s xe2x80x9cAnalysis and Design of Integrated Circuitsxe2x80x9d, J. Wiley and Sons, NY, 3d ed., 1993 at p. 670, the mixer circuit includes two pairs of differentially driven, common emitter-connected transistors connected in series with a further pair of differentially driven, common-emitter connected transistors all of which are connected in series with the emitter biasing resistors across the collector voltage battery. Because sufficient voltage xe2x80x9chead-roomxe2x80x9d must be available for the operation of the mixer only a small portion of the supply voltage remains from the emitters of these transistors to ground for the emitter biasing or linearizing resistor.
As explained, for example, in Gray and Meyer""s text at pp. 715 et seq., of the various sources of circuit noise, thermal noise is a temperature dependent phenomenon that is present in any linear, passive resistor. The amount of thermal noise is independent of the amount of DC current passing through the resistor. Thus, for a given temperature, the signal to noise ratio in resistor is directly proportional to the magnitude of the signal current through the resistor. Conversely, for a given signal current, maximum signal to noise ratio is achieved by increasing the value of the resistor, thereby increasing the value of the signal voltage developed across the resistor. However where, as in the Gilbert multiplier, the emitter resistor is left with only a fraction of the collector battery voltage, an optimum signal to noise ratio cannot be achieved.
We have discovered that the signal to noise ratio of a four-quadrant mixer can be substantially increased by separately differentially driving the pairs of mixer transistors (B1-B4) and stabilizing their common-mode input voltages so that a desired value of current can be established in their common emitter biasing resistors that is independent of the common-mode voltage of the modulating signal source.
The four-quadrant mixer transistors are driven from the collectors of two pairs of local oscillator input buffer transistors (B5-B8) whose pairs of collector resistors are supplied from the common-mode stabilized outputs of an operational amplifier that supplies the differential modulating signal. In this way a larger value emitter biasing resistor can be used than would be possible in a typical Gilbert Mixer for the same value of emitter current or, conversely, larger values of emitter current can be specified to establish a desired level of signal to noise ratio.
A mixer thus configured has substantially low noise by (a) moving the dominate noise sources in front of an operational amplifier where the noise can be filtered and (b) reducing the noise contribution of the few components remaining after the output of the operational amplifier.